This invention relates to a cathode ray tube containing an optical window in the funnel portion thereof, and more particularly, relates to such a tube having the inner surface of the window covered with a layer of transparent conductive material, and also relates to a method for producing such a tube.
In certain specialized applications, it is desirable to have one or more optical windows in the sidewall or funnel portion of a cathode ray tube (CRT), for example, to observe or measure the effects of radiation from the phosphors inside the CRT. The placement of such windows in the CRT presents some special problems for the tube designer. For example, a uniform electrical potential must be maintained between the last element (convergence cup) of the electron gun in the neck of the CRT and the mask-viewing assembly in the forward portion or face panel of the CRT. This is accomplished by applying such potential (via an anode button in the funnel sidewall) to an electrically conductive coating which completely covers the interior surface of the funnel from the mask area to the area of the convergence cup in the neck.
This conductive coating, known as Aquadag or Dag, is usually applied as a suspension of graphite and iron oxide particles in a coating vehicle, and when dry is optically opaque. Because of the high potentials of modern color CRTs, e.g., 20 to 30 KV, the Dag coating desirably exhibits sufficient resistivity to dissipate stray currents caused by arcing, stray emissions and other sources. Conventional Dag coatings exhibit resistivities in the range of 200 to 1000 ohms.
Electrical connection between the Dag and the mask and the Dag and the cup are usually accomplished by means of clips, snubbers or similar spring-type contactors. Connection of the mask to the screen is accomplished by means of a vapor deposited layer of aluminum completely covering the screen surface and screen panel sidewalls at least to a point providing electrical continuity with the mask through the mask's mounting brackets. Such an arrangement results in a space of uniform potential between the gun, mask and screen, enabling precise control of the paths of the electron beams emanating from the gun, and passing through the mask to impinge upon and excite the phosphor elements on the screen.
Since glass is an insulator, it can become charged, distorting the intended beam trajectories. It can thus be readily appreciated that any discontinuity in the internal conductive coating, such as a window, could cause perturbations in the potential field and thus deleteriously effect the deflection paths of the electron beams, with consequent degradation of the viewing screen image.
Electrical continuity could be provided by means of a transparent conductive film across the window area. Glass articles have been made conductive by heating the glass to its softening point, (eg., 500.degree. to 675.degree. C.) and spraying or fuming a solution of tin chloride with a small amount of antimony chloride onto the hot surface. Tin-antimony oxide is thus formed on the surface of the glass. However, unless the HCl (formed as a by-product of the hydrolysis of the metal chlorides) is carried away, such as by a stream of air, the surface of the glass may become clouded or white. Even if the HCl fumes were removed, there is always the danger of residual chloride which could poison the cathode and cause premature failure of the tube. Moreover, because the glass had to be heated to its softening point, glass pitting or distortion may occur. Also, application by spraying or fuming risks thermal shock to the heated glass and is difficult to confine to the desired area.
Other metal oxide films have been applied as transparent coatings on glass, by the pyrolysis of metallo-organic compounds. See, for example, U.S. Pat. No. 3,481,758, which describes such a coating for use as an optical filter on a lens. However, the electrical properties of such coatings vary widely and are of relatively little interest in such applications.
It is, therefore, an object of this invention to provide a CRT containing one or more optical windows in the funnel portion thereof without interrupting the electrical continuity of the Dag coating, by providing a stable adherent transparent conductive coating over the window area, such coating exhibiting an electrical resistivity which is substantially compatible with that of the Dag coating, and making electrical contact therewith.
It is also an object of the invention to provide a method for applying such a coating which is both substantially reliable and compatible with existing CRT manufacturing processes.